1. Field of the Invention:
The present invention relates to a display device which performs its display function by applying driving signals to pixel electrodes via switching elements, and more particularly to an active matrix driving type display device which performs high-density display by using pixel electrodes arranged in a matrix pattern.
2. Description of the Prior Art:
In liquid crystal display devices, EL display devices, plasma display devices and the like, it is known how to produce a display pattern on a screen by selectively driving pixel electrodes arranged in a matrix pattern. In such display devices, voltage is applied between each selected pixel electrode and a counter electrode disposed facing it, to optically modulate a display medium such as liquid crystal or the like interposed between these electrodes. This optical modulation is recognized as a display pattern. As a method for driving pixel electrodes, an active matrix driving method is known in which independent pixel electrodes are arrayed and are driven via switching elements connected to the respective pixel electrodes. As the switching elements used to selectively drive the pixel electrodes, TFT (thin film transistor) elements, MIM (metal-insulator-metal) elements, MOS transistors, diodes, varistors, etc. are generally known. Because the active matrix driving method is capable of performing high contrast display, it has been put to practical use in liquid crystal televisions, word processors, computer terminal displays and the like.
FIGS. 21 and 22 are plan views of active matrix substrates used in active matrix display devices according to a prior art. On the substrate shown in FIG. 21, source bus lines 23 are disposed in parallel and intersecting at right angles with gate bus lines 21 which are arranged parallel with each other. A pixel electrode 41 is disposed in each rectangular area surrounded by two gate bus lines 21 and two source bus lines 23. On each gate bus line 21 and adjacent to the intersection of the gate bus line 21 and the source bus line 23, there is formed a TFT 31 which functions as a switching element, a portion of the gate bus line 21 functioning as the gate electrode of the TFT 31. The drain electrode of the TFT 31 is electrically connected to the pixel electrode 41, while a branch line branching from the source bus line 23 is connected to the source electrode of the TFT 31.
The active matrix substrate of FIG. 22 is the same as that of FIG. 21 except that the construction around the TFT 31 is different. In FIG. 22, the TFT 31 is formed on a gate bus branch line 22 branching from the gate bus line 21, a portion of the gate bus branch line 22 functioning as the gate electrode of the TFT 31.
To achieve high-density display using such display devices, it is necessary to array a great number of pixel electrodes 41 and TFTs 31. However, there may be cases in which some TFTs 31 have been already formed as defective TFTs when they are formed on a substrate. The pixel electrodes connected to such a defective TFT cause defective pixel elements that do not contribute to the display. Such defective pixel elements result in substantial damage to the image quality of the display device, and therefore, greatly reduces the product yield. Display devices having constructions to correct the defective TFTs are disclosed in Japanese Laid-Open Patent Publications Nos. 2-153324, 2-294623 and 2-254423. These display devices comprise spare TFTs which are connected to a pixel element electrode connected to the defective TFT.
There are two major cases of causes for the defective pixel elements. One is the failure to sufficiently charge the pixel electrode during the period in which the pixel electrode is selected by a scanning signal (hereinafter referred to as the "on-failure"). The other is a failure that causes the charge in the charged pixel electrode to leak during the period in which the pixel electrode is not selected (hereinafter referred to as the "off-failure"). The on-failure is attributable to a defective TFT. The off-failure is caused either by electrical leakage via the TFT or by electrical leakage between the pixel electrode and the bus line. In either causes of failure, since necessary voltage is not applied between the pixel electrode and the counter electrode, a defective pixel element is produced. When such failures occur, the defective pixel element appears as a bright spot in the normally white mode in which light transmittance is the highest when the voltage applied between the pixel electrode and the counter electrode is 0 V, and as a black spot in the normally black mode in which light transmittance is the lowest when the voltage is 0 V.
Such a defective pixel element can be corrected by performing laser trimming, etc. However, correction of the defective pixel element must be done on an active matrix substrate before the substrate is assembled into a display device. It is easy to detect pixel defects after the display device has been assembled, but it is extremely difficult to detect pixel defects in an active matrix substrate before assembly, particularly in the case of a large-size display device having 100,000 to 500,000 pixels. Even if the detection and the correction of the defective pixel electrode can be achieved, it requires a high precision measuring instrument as disclosed in Japanese Laid-Open Patent Publication No. 1-144092. It involves a complicated inspection process and is detrimental to mass production efficiency to examine the electrical characteristics of all pixel electrodes and detect the defective TFTs. It therefore causes an increase in costs. For these reasons, as the situation stands now, it is not possible to correct defective pixels in an active matrix substrate before being assembled by the above-mentioned method using a laser beam in the case of large-size display devices having a great number of pixels.